A conventional pilot type fully hydraulic steering system will be described with reference to FIG. 13.
In the same figure, if a wheel H is turned from a straight position H.sub.2 to a left turning position H.sub.1 or to a right turning position H.sub.3, a directional control valve b is changed over from a straight position b.sub.2 to a left turning position b.sub.3 so that an oil under pressure supplied through a hydraulic pump A is supplied to a metering unit c and returned to the directional control valve b. In the case of left turning, a directional control valve d of a steering valve is switched over to a left turning position d.sub.1 so that the oil under pressure from hydraulic pumps M (steering pump) and N (switching pump) is supplied to a head side D.sub.h of a left steering cylinder D and a bottom side E.sub.b of a right steering cylinder E. In the case of right turning, the oil under pressure is supplied to a bottom side D.sub.b of the left steering cylinder D and a head side E.sub.h of the right steering cylinder E so that vehicle bodies F and G are turned relative to each other, whereby the steering operation is made. The oil under pressure, which is returned from left and right steering cylinders D and E, is returned to a tank T through the directional control valve b. Designated at L is a working machine valve P.sub.1 and P.sub.2, which are denoted at two dotted and one chain line and show that the directional control valve b and the metering unit c are operated by the wheel H.
In the conventional pilot type fully hydraulic steering system, it is necessary to correct the deviation of the wheel position since the turning angle of the wheel and the turning angle of the vehicle body do not always accord with each other at the same ratio because of leakage of the oil under pressure inside each hydraulic device, indifference of the steering valve to a pilot oil and revolution of turning of the steering wheel.
Accordingly, the present invention is to eliminate the drawbacks of the conventional pilot type fully hydraulic steering system and to provide a wheel position correcting device capable of eliminating such a drawback that the steering of the wheel is deteriorated due to the gradual deviation of the wheel position when the vehicle body travels straight.